HTZ acknowledges support from the National Natural Science Foundation of China (NNSFC, grants 60704041 and 91023034) and Chinese Program for New Century, MZQC from Nanjing University of Science and Technology (grant AB41680 and Zijin grant AB41356) and T Z from NNSFC (grant 10635040 and 91024026). CZ is supported by Hong Kong Baptist University.